1. Field of the Invention
The present invention relates to a radio communication apparatus for performing transmission using selective diversity and a transmission antenna changing method.
2. Description of the Related Art
In a radio communication system, there has recently been used space diversity having a plurality of antenna branches (hereinafter referred to as simply "antenna") in a base station, for securing a plurality of paths in order to improve the quality of reception. As one space diversity, there is a selective diversity that selects an optimum antenna in accordance with a propagation state.
In the case of using the selective diversity on the transmit side, in a reverse link, the received signal strength of each terminal is obtained for each antenna and an antenna with a large received signal strength is selected, and signals are transmitted from the selected antenna.
Hereinafter, a description is given of the flow of signals in a conventional radio communication apparatus using a base station as an example of a radio communication apparatus. FIG. 1 is a block diagram showing the configuration of the conventional base station.
First of all, signals radio transmitted from the respective terminals are received by antennas 11 to 14 at the reception timing, and the received signals are input to received signal strength detecting means 19 and 20 through transmission/reception switches 15 to 18.
Then, received signal strength of each antenna of each terminal is detected by detecting means 19 and 20, and each detected received signal strength is associated with each antenna number, and the result is stored temporarily.
After that, a transmission antenna selector 21 selects an antenna with a maximum received signal strength for each terminal and outputs a corresponding control signal to a transmission antenna change device 24, so that an internal switch of transmission antenna change device 24 is changed on the basis of the control signal.
Then, at the transmission timing, transmitting signals with respect to each terminal are output from transmitters 22 and 23, and input to any one of mixers 25 to 28 through transmission antenna change device 24.
Thereafter, signals input to the respective mixers 25 to 28 are multiplexed, and radio transmitted from any one of antennas 11 to 14.
FIG. 2 is a view showing a result of transmission antenna select processing, which is carried out by the transmission antenna selector 21 of the conventional base station.
In FIG. 2, regarding the received signal strength of antennas of terminal A, antennas 11 to 14 have 47 dB, 40 dB, 42 dB, 37 dB, respectively. Also, regarding the received signal strength of antennas of terminal B, antennas 11 to 14 have 50 dB, 45 dB, 38 dB, 41 dB, respectively.
In the case of FIG. 2, the antenna with a maximum received signal strength of terminal A is antenna 11, and the antenna with a maximum received signal strength of terminal B is antenna 11. Therefore, transmission antenna selector 21 selects antenna 11 as a transmission antenna for terminal A, and selects antenna 11 as a transmission antenna for terminal B.
Thus, in the aforementioned conventional radio communication apparatus and the transmission antenna changing method, an antenna whose received signal strength becomes maximum at the reception timing of pre-transmission is selected as a transmission antenna.
However, in the aforementioned conventional radio communication apparatus, mixers are required since there is possibility that transmitting signals will be transmitted to a plurality of terminals from the same antenna, and if n (n=two or more natural number) transmitters are used, n mixers are needed. For this reason, the more the number of transmitters to be used is increased, the greater a loss (about 3 dB.times.n) occurred in the mixers becomes, so that unnecessary heat is generated.
Also, the loss occurred in the mixers corresponds to about a half of energy generated by a transmission amplifier provided in the transmitter, and this causes a problem in which a large-sized transmission amplifier must be used in consideration of such a loss.